The Tau Empire originates from the far Eastern Rim of the known galaxy. They are the youngest known race, but have expanded their technology far faster than any other. The core belief of the Empire is for them to fight for the Greater Good, the idea that the survival of their race and their allies’ is above the needs of the few. Because of this, they form a strong willed, well armed force that wants expansion and will take it from whoever gets in their way. But on occasion, short truces have been made until a larger threat has been defeated.
The Tau Empire functions on a caste based society, each stemming from their five separate subraces that evolved on their homeworld of Tau. Those of the Fire Caste are the ones that most of the Imperium is familiar with. They are strongest of their race and bred for combat prowess. The Earth Caste is the shortest and stoutest of them all, their craftsmen and artisans. The Air Caste are the tallest and frailest, a full head taller than most the race and with hollow bones, they are the pilots and navy. The Water Caste are bred to be their bureaucratic and merchant caste. Finally, there is the Ethereal Caste, their philosopher kings that rule over each planet. Very few Ethereals are born, and generally there are only one or two per planet.
The Tau Empire is also the only galactic force that actively employs Xenos aid in military matters. They have made use of many different auxiliaries under their aid. Most notable are the Kroot, the Vespid, and the Demiurg, the latter giving them the ability to travel through the Warp.